This invention relates generally to imaging apparatus and methods, and more specifically relates to imaging systems and methods which utilize electromagnetic radiation in the Terahertz (THz) range as incident energy at objects to be examined.
The present invention is directed toward detection of weaponry concealed on the person, or in containers such as suitcases, briefcases, sealed packages, or cartons. Such weaponry can include metallic devices such as handguns, but the invention especially addresses the issue of monitoring, detecting and characterizing concealed explosives and biological weapons.
As plastique explosives, fertilizer bombs and biological agents increasingly become weapons of war and terrorism, effective means for rapid detection and identification of concealed caches of these agents is increasingly imperative. One proposed solution is to use terahertz (THz) electromagnetic waves to spectroscopically detect and identify concealed explosives and biological weapons through their characteristic transmission or reflectivity spectra in the THz range (0.1-10 THz). Explosives (e.g. C-4, HMX, RDX, TNT, naphthalene, and ammonium nitrate) all have characteristic reflection and absorption spectra in the 0.1-2.0 THz range (100-2000 GHz, 3-0.15 mm) which are easily distinguishable from other materials such as human skin. In essence, explosives appear as different xe2x80x9ccolorsxe2x80x9d to the THz detector as compared to non-hazardous items. The use of THz for the detection of biological weapons has also shown great promise. Using THz spectroscopy it is therefore possible to in principle detect explosives and biological weapons even if they are concealed in clothing, sealed packages, suitcases, etc since the THz radiation is readily transmitted through plastics, clothing, luggage, paper products, walls, and other non-conductive (non-metallic) materials. By comparing measured reflectivity (or transmitted) THz spectra with known calibration spectra, one may therefore identify the presence of these agents and distinguish them from objects such as keys, coins, human skin, and clothing. Since metals are relatively opaque to transmission of THz wavelengths and have a roughly constant reflection spectra, metal weapons such as handguns and knives are similarly identifiable by THz examination.
Most THz imaging systems proposed in the past have been based upon a single THz source and detector pair that are scanned across the object space to be imaged. These systems consequently take a significant amount of time (typically minutes) to acquire the data to generate a THz image of even a single small object (e.g. of approximately a few square centimeters), and are not suitable to real-time acquisition of THz images. Additionally, current state-of-the-art THz imaging is based on short-pulsed laser or continuous wave difference frequency THz generation and detection. The difficulty with extending either of these techniques to continuous wave THz imaging of coherent or incoherent THz radiation is that coherent continuous wave or short-pulsed laser sources are required. Moreover, the laser sources that generate and detect the THz radiation must retain a coherent phase relationship to each other. Using these methods, the imaging of an incoherent THz source is not possible. The present invention design and technique does not require a particular coherent or incoherent source of THz. It allows the flexibility to utilize an electronic THz source, a laser-based THz illuminating source, or incoherent ambient THz radiation which might be present, for example, from the sun.
An object of the present invention is to provide a spatial THz imaging technique which is capable of detecting multiple THz sources simultaneously within a wide field-of-view. To accomplish the same functionality with a single line of sight measurement, the line-of-sight has to be scanned across the field of view to be measured, which incurs the difficulties discussed above. By means of the present invention the spatial resolution is sufficient not only to determine that an explosive or biological agent is present, for example, but also the physical extent and location of the object. This information is difficult to determine with a line-of-sight technique. The THz imaging approach of the present invention has sufficient spatial resolution to detect explosives or biological agents that are concealed on a person or hidden in packages, containers or vehicles from a stand-off distance. A longer term advantage of the present invention is that it produces more information than that yielded by a single line-of-sight system. By obtaining multiple images over time one can apply imaging processing techniques to multiple images and multiple THz sources that can be used to reject noise and reduce false alarms in a complete system.
Now in accordance with the present invention THz imaging apparatus and methods are provided for rapidly and effectively examining a region of interest to determine the presence of specified compositions. The apparatus includes means for generating electromagnetic radiation of a desired terahertz frequency suitable for the examination, and for rendering the radiation incident at the region of interest. Detector means are provided at a plurality of points in a plane spaced from the region of interest, for detecting the terahertz radiation reflected from or transmitted through the region. Means are provided for converting the detected terahertz radiation to an image of the region of interest from which the presence of the specified compositions are determinable.
For routine, stand-off sensing, a wide area is illuminated with a bright THz source. The source can be broadband and incoherent (such as radiation from the sun) or narrow band and tunable. The transmitted or reflected THz radiation is then detected with a THz imaging array. A critical technical limitation to this approach has in the past been the lack of an imaging THz detector array (regardless of spatial resolution or tunability). Crudely speaking, the equivalent of a digital camera has not existed in the THz regime. The limitation in the THz regime has not been the xe2x80x9ccameraxe2x80x9d lens but rather the detector array that digitizes the image.
The THz imaging array of the present invention is related to our previous work in radio astronomy. Unlike radio astronomy for which the positions and spectral content of the radio sources (stars) is not known before hand, the spectral content and location of sources is known for THz-based standoff detection of explosives/biological agents, but the THz transmission properties of the intervening objects needs to be determined.
In the present invention, the detector means may comprise a tunable interferometric array of spaced detectors. The signal outputs from pairs of the detectors are combined with proper delay and correlation in phase and quadrature to produce components for the Fourier transform plane corresponding to the detector plane. The detector array can include a plurality of semiconductor photomixers. Photomixer driving means comprising a frequency stabilized tunable optical heterodyne source are coupled to the photomixers by a common fiber optic connector. The array can comprise an in-line arrangement of detectors, and means for rotating the array about a fixed axis. The original brightness distribution at the region of interest are recovered by Fourier inversion of the Fourier components.
The photomixers can be photoconductive devices, wherein the driving means for each pair of such devices is a pair of lasers having a difference frequency which gates the photomixers, the incoming terahertz radiation at each member of the pairs of photomixers being mixed with the difference frequency to provide modified signal outputs at intermediate frequencies in order to facilitate signal processing.
The wide field-of-view THz imaging interferometer array used is capable of high spatial resolution and spectral resolution in the 0.2-10 THz range. This array can image multiple sources of coherent or incoherent THz light simultaneously without the need for expensive short-pulsed laser systems. The invention thus may include a continuous wave terahertz imaging spectrometer tunable from 0.2 to 3 THz in order to remotely detect, monitor and identify concealed explosives such as C4 and RDX. The system utilizes a high brightness THz source illuminating the region of interest and detected by the tunable THz interferometric imaging array. The THz imaging array has a wide field of view and high spatial resolution. While the system preferably uses a heterodyne photomixing detection technique, it may also use homodyne photomixing detection. The high-speed photomixing devices for heterodyne detection is preferably designed to operate at optimum intermediate frequencies. The photomixing devices can serve both as source and detectors. A database of required THz-frequency spectral signatures for target explosives is provided for use with the system, and neural network algorithms can also be conjunctively used to identify selected explosives from the THz images.